1. Field of the Invention
The present invention relates to a wireless personal area network (WPAN). More particularly, the present invention relates to a packet transmission method for a WPAN that alleviates traffic concentration in an active period and improving network throughput.
2. Description of the Related Art
Recently, computer networks have been evolving to support various information processing devices. With the advance in network technologies, wireless networks are replacing conventional wired networks. These wireless networks are advantageous in mobility and eliminate the need for extensive cabling and patching. Wireless networking technologies can be classified into Wireless Local Area Network (WLAN) technologies supporting wireless connections in the range of 50˜100 m and Wireless Personal Area Network (WPAN) technologies supporting wireless connections up to 10 m. However, when considering a home network for interconnecting the electric appliances, WPAN technologies are suitable in view of ad hoc networking capability.
In such a wireless network, a network coordinator manages other devices. Moreover, there is a plurality of network coordinator-based WPAN standards developed by IEEE 802.15 Working Group.
In such a WPAN environment, the network nodes including a network coordinator and other devices often operate with battery power, meaning that low power consumption is one of the most important design constraints. High energy efficiency can be achieved with a low duty cycle (a ratio of active period to entire period) operation. There is a research showing that it is required to set the duty cycle of a network device to below 1% in order to use a battery having the most popular capacity for 2 years.
In the meantime, IEEE 802.15.4 intends to offer a Low Rate WPAN (LR-WPAN) which focuses on low cost, low speed ubiquitous communication between devices. The IEEE 802.15.4 task group (ZigBee®) deals with low data rate but long battery life and very low complexity for use in applications such as wireless remote controller, electric appliance controller, building control, toy, and the like. In 2003, the standardization of the physical (PHY) layer and medium access control (MAC) layer has been completed. The data rate is 250 kbps at 2.4 GHz (16 channels), 40 kbps at 915 MHz (10 channels), and 20 kbps at 868 MHz (1 channel) with connection range of 1˜100 m.
As mentioned above, the IEEE 802.15.4 WPAN operates at a low duty cycle for minimizing power consumption. However, the low duty cycle based operation causes some problems.
FIG. 1A is a diagram illustrating a traffic distribution of a conventional IEEE 802.15.4 WPAN operating with a medium duty cycle, and FIG. 1B is a conventional diagram a traffic distribution of an IEEE 802.15.4 WPAN operating with a low duty cycle.
As shown in FIGS. 1A and 1B, as the duty cycle decreases, a number of devices preparing transmission of packets increases during the inactive period (identified by the dashed circles) such that the more packets are attempted to be transmitted in the next active period. In the conventional packet transmission method, the devices content with a fixed initial backoff value without consideration of whether the packets are generated in the active or inactive period. This causes traffic concentration in the active period and, in turn, increases packet collision probability and packet loss.